President Obama has pledged to reduce the number and role of nuclear weapons in America's national security strategy. In a statement to mark the 40th anniversary of the nuclear non-proliferation treaty, Mr. Obama said his administration would seek to ratify the comprehensive nuclear test ban treaty and work to negotiate a treaty to end the production of the radio active materials used to make nuclear weapons.<br/><br/> Mr. Obama will host a nuclear security summit in Washington next month. His officials are also involved in talks with Russia on a replacement for the 1991 Strategic Arms Reduction Treaty (Start), which expired last December. <br/><br/> The treaty, which bans all nuclear explosions, was adopted by the UN in 1996 but has not yet entered into force.<br/><br/> President Obama laid out his vision for a nuclear-free world in a keynote speech last year in the Czech capital, Prague.